The invention relates to a filter comprising a shell construction with a wall construction and a bottom portion forming at least one section in the shell construction, said filter further comprising at least one resonator within a section of the shell construction.
The invention also relates to a method for manufacturing a filter comprising producing a shell construction with a wall construction, a bottom portion and at least one section, and at least one resonator in said shell construction.
Radio frequency filters are used for implementing high-frequency circuits for instance in base stations of mobile telephone networks. Filters can be used, for example, as interface and filtering circuits in the amplifiers of transmitter and receiver units in base stations.
There are several different types of resonator filters comprising a shell construction, or body, e.g. coaxial resonator filters. In coaxial resonator filters, the shell envelops a conductor which is positioned in a section of the shell and which is called a resonator or resonator pin. High-frequency filters, for example, particularly more complicated filters, are provided with a multi-section shell construction and so-called subdivision. In this case, the resonator filter has a multi-section, or multi-cavity, shell construction; in other words, it comprises a plurality of resonator cavities, or sections in the shell construction, each of which forms a separate resonant circuit with the corresponding resonator.
In some known coaxial resonator filters, the shell construction and the resonators are made of separate pieces, wherefore the resonators must be, for instance, soldered onto the bottom of the shell construction. Such a construction increases the probability of detrimental intermodulation and is slow to manufacture. There are also known solutions in which material is milled away from a sufficiently large metal block so that the remaining part of the block constitutes the shell construction and resonator pins of the filter. Such a solution consumes a great deal of raw material and requires time-consuming manufacturing steps.
U.S. Pat. No. 4,706,051 discloses a solution according to which halves of a waveguide shell construction are manufactured by forging into a die: a slug of material is hit by a punch such that the slug material is displaced in the closed space between the die and the punch. This publication does not disclose any solution for manufacturing resonators. The solution according to this publication has drawbacks, since it involves the manufacture of complementary halves of a shell, and since the slug material displaced as a result of punching to form a half of the shell construction does not flow freely, since the flow of the material is restricted by the closed die.
U.S. Pat. No. 5,329,687 discloses a solution according to which both a shell construction and a resonator are moulded or extruded from plastic as an integral unit to be coated with metal. However, the thermal conductivity of such a construction is not good. In addition, U.S. Pat. No. 4,278,957 discloses a solution according to which resonators are cast in the shell construction. The construction of the last-mentioned publication is manufactured by die casting, which requires a multielement die arrangement which must open in at least three directions. On account of the material residues left in the joints of the die, a resonator made by die casting will not be entirely circular, which impairs the electrical properties of the resonator.